The present invention relates to radio frequency coil design and construction. It finds particular application in conjunction with quadrature coils for use in magnetic resonance imaging systems and will be described with particular reference thereto. However, it is to be appreciated that the present invention will find application in magnetic resonance spectroscopy, and other fields in which the isolation of multiple coils or the angle of magnetic field vector is adjusted or calibrated.
Heretofore, magnetic resonance imaging and spectroscopy equipment has utilized quadrature reception coils to boost signal strength. Commonly, the quadrature coils include two pairs of oppositely disposed saddle coils. The pairs of oppositely disposed saddle coils were rigidly mounted offset precisely 90.degree. relative to each other. When the pairs of coils were exactly the same and mounted exactly 90.degree. apart, the radio frequency signals received by each pair were isolated from each other. However, even a small difference in the coils or deviation from a precise 90.degree. relationship resulted in a loss of isolation and a rapid decrease in the signal strength.
One common technique for isolating the quadrature coils has been to distort the electromagnetic field with shims or paddles. That is, metallic elements were disposed adjacent the quadrature coils in sufficient proximity that the magnetic fields were distorted. The position and orientation of the paddles were adjusted until isolation of the two coil pairs were maximized. One of the problems with paddles is that they modified or distorted the magnetic field pattern. The modification of the field pattern were relatively unpredictable. Conversely, the location in which to place a paddle to effect a selected correction was also unpredictable. This lack of predictability caused the manufacturing and final calibration to be time consuming and difficult.
In another tecnique, each coil pair was mounted on a separate, circularly cylindrical dielectric former. The formers were concentrically mounted such that one pair of coils could be rotated relative to the other pair. The position or rotation of the formers were adjusted until the isolation was maximized. One of the problems with the concentric former technique was the complexity of achieving and maintaining rigid, precise alignment. In a 30 centimeter coil, a linear movement of 0.5 mm, which is equivalent to 0.15.degree. of rotation caused a loss of effective coil isolation of 40 to 20 db.
It has also been suggested that the isolation might be adjusted by connecting a two part capacitive network to the feed points of the two coils. However, connecting the two port capacitive network to the relatively spaced feed points of the coils tended to create stray effects, particularly at higher frequencies.
In accordance with the present invention, a new and improved technique is provided for electrically adjusting the isolation between quadrature or other coil pairs.